Crosshole GPR surveys are able to quantify in spatial detail: 1) the initial density state of sandy fill at Treasure Island, and 2) the changes in density due to a liquefaction event. Volumetric strain associated with these changes in density would result in settlements of 17 cm for the 4-m-thick liquefied layer estimated for the site. The radar-based estimates of settlement are remarkably close to ground-level changes observed at the site. This experiment demonstrates the applicability of crosshole GPR for non-destructive imaging of void ratio within a large volume of soil, and for characterizing changes in soil volume due to a liquefaction event.